C语言中的 @ 符号是什么意思?

2021-03-10 05:29

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标签：assign inter res ted 访问内存变量出现 read_only his

Global Variable Address Modifier (@address)
You can assign global variables to specific addresses with the global variable address modifier. These variables are called ‘absolute variables‘. They are useful for accessing memory mapped I/O ports and have the following syntax:

Declaration = [@|@"

"]
[= ];
is the type specifier, e.g., int, char

is the identifier of the global object, e.g., i, glob

is the absolute address of the object, e.g., 0xff04, 0x00+8

is the value to which the global variable is initialized.

A segment is created for each global object specified with an absolute address. This address must not be inside any address range in the SECTIONS entries of the link parameter file. Otherwise, there would be a linker error (overlapping segments). If the specified address has a size greater than that used for addressing the default data page, pointers pointing to this global variable must be "__far". An alternate way to assign global variables to specific addresses is (Listing 8.8).

Listing 8.8 Assigning global variables to specific addresses

#pragma DATA_SEG [__SHORT_SEG]
setting the PLACEMENT section in the linker parameter file. An older method of accomplishing this is shown in Listing 8.9.

Listing 8.9 Another means of assigning global variables to specific addresses

INTO READ_ONLY ;
Listing 8.10 is a correct and incorrect example of using the global variable address modifier and Listing 8.11 is a possible PRM file that corresponds with example Listing.

Listing 8.10 Using the global variable address modifier
//看这意思,就是把int型变量glob地址从0x0500开始,并把值10初始化时放在0x0500
int glob @0x0500 = 10; // OK, global variable "glob" is
// at 0x0500, initialized with 10
void g() @0x40c0; // error (the object is a function)

void f() {
int i @0x40cc; // error (the object is a local variable)
}

全局变量地址修饰符（@address）
您可以使用全局变量地址修饰符将全局变量分配给特定地址。这些变量称为“绝对变量”。它们对于访问内存映射的I / O端口很有用，并具有以下语法：

声明= [@ | @“ ”]
[= ];
是类型说明符，例如int，char

是全局对象的标识符，例如i，glob

是对象的绝对地址，例如0xff04、0x00 + 8

是全局变量初始化到的值。

将为每个用绝对地址指定的全局对象创建一个段。该地址不得在链接参数文件的SECTIONS条目中的任何地址范围内。否则，将出现链接器错误（重叠段）。如果指定的地址的大小大于用于寻址默认数据页的大小，则指向此全局变量的指针必须为“ __far”。将全局变量分配给特定地址的另一种方法是（清单8.8）。

清单8.8将全局变量分配给特定地址

#pragma DATA_SEG [__SHORT_SEG]
在链接器参数文件中设置PLACEMENT部分。清单8.9显示了完成此操作的较旧方法。

清单8.9将全局变量分配给特定地址的另一种方法

INTO READ_ONLY ;
清单8.10是使用全局变量地址修饰符的正确和不正确的示例，清单8.11是与示例清单相对应的可能的PRM文件。

清单8.10使用全局变量地址修饰符
//看这意味着，就是把int型变量glob地址从0x0500开始，并把值10初始化时放在0x0500
int glob @ 0x0500 = 10; //确定，全局变量“ glob”为
//在0x0500处，以10初始化
无效g（）@ 0x40c0; //错误（对象是一个函数）